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The History of the Tatara

Tessai (mineral composition of slag)

Much data can also be obtained regarding the mineral composition of iron slag by examining it with a microscope.
Smelting slag is composed primarily of fayalite (Fe2SiO4) and silicate glass substances. When iron sand high in titanium (i.e., akome) is used, the slag will contain crystals of mineral titanium, which is composed of ulvospinel (Fe2TiO4) and ilmenite (FeTiO3). Based on the state of these crystalline structures, it is possible to make estimates regarding the temperature and rate of cooling at which they were produced.

In the case of okaji slag (also called refining smithing slag), the volume of Ti inclusions are extremely small, and there is almost no crystallization of mineral titanium, even with slag where iron sand was used. Instead, when the presence of iron in the makeup is high, at more than 50%, the volume of manufacturing slag is also lower, and the iron oxide called wuestite (FeO) in the mineral structure is crystallized. Dendritic crystals of wuestite will appear in small amounts even in kera-oshi smelting slag. It forms an intermediate structure in zuku-oshislag and okaji slag.

Wuestite is present in large amounts in kokaji slag (also called forging smithing slag), producing another level. Also, small scraps of iron (oxidized scale from the iron) produced during forging accompany the slag.

Shown for reference below are the typical mineral structure of iron slag and a chemical analysis of smithing slag.

Microscope photos of the mineral structure of typical iron slag
(all photos are at the same magnification)

Figure: Chemical composition of typical smithing slag (mass %)
NOTES: (1) Kanayago tatara; (2) Yasukuni tatara; (3) Mt. Tsugo tatara

We can glimpse of one aspect of ancient iron manufacturing through this type of research on iron slag. As a result, we know that the manufacture of iron using prospered on the San'yo side of the region in ancient times. We know also that refining and smithing (okaji) took place during the Yayoi Period (B.C.E. 330 to 330 C.E.), but that iron manufacturing did not. It is also clear that iron manufacturing began to take place from around the 6th century C.E.

However, this is all based on conjectures made about ancient techniques that rely on data about iron slag obtained through the use of post-Meiji Period tatara technology. If the ancient technologies differed significantly from those in use since the Meiji Period, then our presumptions will have been mistaken. For example, we don't know about the iron slag produced when smelting in a furnace where both the temperature and reducibility are low. It might have the intermediate characteristics of kera-oshi and smithing slag, and someone might decide that it is smithing slag. In the future, it will be necessary to conduct research on iron slag produced through such basic iron-making methods.

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